The material used for the separator of a fused carbonate fuel cell is required to exhibit a degree of corrosion resistance sufficient to be safe from fused-salt corrosion by the fused carbonate electrolyte of the cell. The practice has therefore been to use a high-grade stainless steel such as SUS316L or SUS310S as the material for the separator of a fused carbonate fuel cell.
Since the fused-salt corrosion resistance of a stainless steel is known to be generally proportional to its Cr content, high Cr content stainless steels such as SUS310S are the most frequently used. Stainless steels with high Cr content also include a large amount of Ni for stabilizing the austenite phase.
Such steels have also been used as separator material after being imparted with high corrosion resistance by coating with alumina.
Technologies for improving the resistance of stainless steels to corrosion by fused carbonate are taught by, for example, JPA-62-294153 (Separator Material for Fuel Cells) and JPA-1-252757 (Fe-base Alloy Excellent in Fused-Carbonate Corrosion Resistance). The first of these, JPA-62-294153, teaches improvement of fused-carbonate corrosion resistance by addition of Al to the steel.
When a large amount of Al is added to a stainless steel for increasing resistance to fused salt, however, the hot workability of the steel is markedly degraded and large cracks tend to occur during hot rolling for production of steel sheet. Since this restricts the size of the steel sheet that can be produced, lower product yield and higher production cost are unavoidable. For coping with this hot workability problem of austenitic stainless steel containing Al, JPB-55-43498 teaches a method of improving the hot workability of a stainless steel such as by causing precipitation of a small amount of .delta. ferrite during solidification or by addition of a rare earth element such as La or Ce, while JPA-60-262945 proposes a method of preventing cracking by conducting hot rolling at a temperature in the range of 1000.degree.-1200.degree. C.